U.S. patent number 5,223,086 [Application Number 07/848,721] was granted by the patent office on 1993-06-29 for method of producing an acceleration sensor of a semiconductor.
This patent grant is currently assigned to Nippondenso Co., Ltd.. Invention is credited to Minoru Nishida, Masakazu Terada, Shinsuke Watanabe.
United States Patent |
5,223,086 |
Terada , et al. |
June 29, 1993 |
Method of producing an acceleration sensor of a semiconductor
Abstract
This invention relates to a method of producing an acceleration
sensor of a semiconductor. Piezo resistance layers are formed in a
silicon tip 2 of a single crystal etched in an anisotropic etching
liquid such as a KOH solution, etc., using as a mask a silicon
nitride film. Then the silicon tip 2 of the single crystal is
soaked in an isotropic etching liquid for a predetermined time and
is etched to a depth of 0.5-2.0 .mu.m. Further, a photoresist is
applied over a whole surface thereof to form a slot extending to a
hollow.
Inventors: |
Terada; Masakazu (Kariya,
JP), Nishida; Minoru (Okazaki, JP),
Watanabe; Shinsuke (Aichi, JP) |
Assignee: |
Nippondenso Co., Ltd. (Kariya,
JP)
|
Family
ID: |
12713105 |
Appl.
No.: |
07/848,721 |
Filed: |
March 9, 1992 |
Foreign Application Priority Data
|
|
|
|
|
Mar 11, 1991 [JP] |
|
|
3-45217 |
|
Current U.S.
Class: |
438/51; 438/52;
73/514.34; 73/514.36 |
Current CPC
Class: |
G01P
15/0802 (20130101); G01P 15/123 (20130101) |
Current International
Class: |
G01P
15/12 (20060101); G01P 15/08 (20060101); H01L
021/306 (); B44C 001/22 () |
Field of
Search: |
;156/647,651,659.1,662,633,639 ;29/25.35,621.1
;73/488,496,517R,517AV,517A |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
60-154575 |
|
Aug 1985 |
|
JP |
|
62-60270 |
|
Mar 1987 |
|
JP |
|
1-274478 |
|
Nov 1989 |
|
JP |
|
1-302167 |
|
Dec 1989 |
|
JP |
|
3-37749 |
|
Jun 1991 |
|
JP |
|
Primary Examiner: Powell; William A.
Attorney, Agent or Firm: Cushman, Darby & Cushman
Claims
We claim:
1. A method of producing an acceleration sensor of a semiconductor,
characterized by:
performing a 0.5 .mu.m-2.0 .mu.m isotropic etching of a
semiconductor tip of a single crystal after performing an
anisotropic etching, to thereby form a portion of the semiconductor
tip of the single crystal as a thin beam in which piezo resistance
layers are formed.
2. A method of producing an acceleration sensor of a semiconductor
according to claim 1, wherein a mixed liquid of hydrofluoric acid,
nitric acid and acetic acid is used at a predetermined volume ratio
as the isotropic etchant.
3. A method of producing an acceleration sensor of a semiconductor
according to claim 1, wherein a depth of the isotropic etching is
adjusted by varying a soaking time of the isotropic etching.
4. A method of producing an acceleration sensor of a semiconductor
according to claim 1, wherein the semiconductor tip of the single
crystal is accommodated in a can and soaked in a silicone oil.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of producing an
acceleration sensor of a semiconductor.
2. Description of the Related Art
In a conventional acceleration sensor of a semiconductor, a silicon
tip of a single crystal is provided with a thin beam having piezo
resistance layers, and to form this beam, an anisotropic etching
method using KOH, etc. is used, to provide a very high
manufacturing accuracy.
Nevertheless, in the anisotropic etching method since a
concentration of stress often occurs at an edge part of a beam
base, a problem arises in that the strength thereof can not be
guaranteed. Particularly, since in the acceleration sensor of a
semiconductor for detecting a very small acceleration of less than
1 G it is necessary to thin the thickness of the beam as much as
possible, to thus raise the sensitivity, the beam is very week and
therefore, the yield is low.
SUMMARY OF THE INVENTION
A object of the present invention is to provide a method of
producing a high sensitive acceleration sensor of a semiconductor
in which the semiconductor tip has a high beam strength.
In one aspect of the present invention, an 0.5 .mu.m-2.0 .mu.m
isotropic etching is performed on a semiconductor tip of a single
crystal, after an anisotropic etching thereof, to form a portion of
the semiconductor tip of the single crystal as a thin beam in which
piezo resistance layers are formed.
As a result of performing a 0.5 .mu.m-2.0 .mu.m isotropic etching
of a semiconductor tip of a single crystal, after performing an
anisotropic etching, to form a portion of the semiconductor tip of
the single crystal as a thin beam in which piezo resistance layers
are formed, a edge portion of the beam base is rounded to thus
alleviate a stress concentration thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view showing an acceleration sensor of a semiconductor
packaged in a can;
FIG. 2 is a view showing a lower surface of a silicon tip of an
acceleration sensor of a semiconductor;
FIG. 3 is a plane view showing a silicon tip;
FIG. 4 is a view showing a manufacturing process;
FIG. 5 is a view showing a manufacturing process;
FIG. 6 is a view showing a manufacturing process;
FIG. 7 is a view showing a manufacturing process;
FIG. 8 is a view showing a manufacturing process;
FIG. 9 is a view showing a relationship between a beam thickness
and an applied break load;
FIG. 10 is a view showing a relationship between an etching amount
and a break strength increasing rate;
FIG. 11 is a view showing a composition of an isotropic etching
liquid;
FIG. 12 is a view illustrating a method of measuring an etching
amount;
FIG. 13 is a sketch showing a state of a broken position;
FIG. 14 is another sketch showing a state of a broken position;
and,
FIG. 15 is a view showing a relationship between an etching amount
and a broken state occupying rate.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention will be described with reference to specific
embodiments thereof, and the accompanying drawings.
FIG. 1 shows an acceleration sensor 1 with a semiconductor packaged
in a can, FIG. 2 is a lower surface view (seen in the x direction
of FIG. 1) of a silicon tip 2 of a single crystal in the
acceleration sensor 1 of the semiconductor, and FIG. 3 is a plane
view (seen in the Y direction of FIG. 1) of the silicon tip 2 of
the single crystal. This silicon tip 2 of the single crystal is
formed in a cantilever structure, i.e., as a base 3 of a rectangle
loop, a thin beam 4 is provided in the loop interior, and further,
a thick layer 5 is provided at a extremity of the beam 4. Also the
beam 4 is provided with four piezo resistance layers 6, 7, 8, 9,
including a bridge circuit.
FIGS. 4 to 8 show a process of manufacturing the acceleration
sensor of the semiconductor.
First, as shown in FIG. 4, a plate-like silicon tip of the single
crystal is prepared, and impurities are collected at a
predetermined region of the upper surface thereof so that the four
piezo resistance layers 6, 7, 8, 9 (See FIG. 3) are formed.
Thereafter, a plasma.silicon nitride film 10 is formed over the
whole surface of a lower part of the silicon tip of the single
crystal, and further, as shown in FIG. 5, the silicon nitride film
10 is patterned by photo-etching to form the cantilever. Next, as
shown in FIG. 6, a region of the silicon tip 2 of the single
crystal that should not be etched is covered with a ceramic plate
11 and wax 12, the silicon nitride film 10 is used as a mask, and
the silicon tip 2 of the single crystal is etched with an
anisotropic etchant such as a KOH solution, etc. As a result the
silicon tip of the single crystal is provided with a rectangle
loop-like hollow 13, a portion of which becomes the thin beam
4.
When performing this anisotropic etching, as shown by a dashed line
in FIG. 7, edges (shown by P1, P2 in FIG. 7) at the periphery of
the beam 4 are sharp.
Next, after washing with water, the silicon tip 2 of the single
crystal is soaked in the isotropic etchant for a predetermined
time, and as shown by a solid line in FIG. 7, etched to a
predetermined depth. In the present embodiment, an isotropic liquid
such as a mixture of hydrofluoric acid, nitric acid and acetic acid
is used. An adjustment of the depth of the isotropic etching is
made by varying a soaking time thereof in the etchant. The depth of
the isotropic etching is preferably 0.5 .mu.m-2.0 .mu.m. Thereafter
the silicon tip 2 is washed with water.
Further, as shown in FIG. 8, a photoresist 14 is applied over a
whole surface of the silicon tip 2 of the single crystal in such a
manner that it is patterned by a photolithography to thus form the
cantilever. A region that should not be etched is covered with a
ceramic plate and wax 16, before performing the isotropic etching.
As a result, a slot 17 extending to the hollow 13 in a region other
than the beam in the silicon tip 2 of the single crystal is formed
(see FIG. 3), and thus the cantilever is formed.
At this stage, the thickness of the beam 4 of the silicon tip 2 is
several .mu.m to several 10 .mu.m, and the thickness of the base 3
and the layer 5 of the silicon tip 2 of the single crystal is
several 100 .mu.m. The silicon tip 2 of the single crystal is then
placed on a washer 18, accommodated in the interior of a can 19,
and soaked in silicone oil 20.
For measuring the acceleration, when the acceleration is applied to
the acceleration sensor of the semiconductor, the beam 4 of the
cantilever is displaced such that the base thereof becomes a
supporting point whereby a distortion of the beam 4 occurs.
Therefore, a bridge output based on a change of the piezo
resistance value is obtained from the bridge circuit formed by the
piezo resistance layers 6-9. Also, when the acceleration is
applied, the silicone oil 20 inside the can 19 lowers the resonant
frequency, which high frequency prevents the cantilever from
breaking.
In FIG. 9, a cantilever strength is shown by the relationship
between the silicon tips 2 of the single crystal that is etched by
1 .mu.m in the isotropic etching and not etched in the isotropic
etching. From this figure, an improvement of the strength obtained
by performing the isotropic etching is seen to be about twice as
much as that when not performing the isotropic etching. Namely, as
shown in FIG. 7, the edge P2 of the base of the beam 4 in the
silicon tip 2 of the single crystal is rounded to thereby alleviate
the stress concentration thereat.
Further, FIG. 10 shows the results obtained by investigating a beam
strength improving rate and a etching amount (depth) based on
performing the isotropic etching. In this measurement, the
isotropic etching liquid having the composition shown in FIG. 11 is
used. Namely, in volume ratio, hydrofluoric acid:nitric acid:acetic
acid=2:45:3 is used. As seen from FIG. 10, for example, a 0.5 .mu.m
etching amount produces a 1.7 times improvement of the strength.
Further, if the strength improvement rate is more than 1.5 times,
since a poor rate inside a process (when working in a manufacturing
process) may be about 0%, the etching amount may be more than 0.25
.mu.m, but after considering the scattering during etching, it is
found to need a more than 0.5 .mu.m etching. Also, in the case of a
more than 2 .mu.m etching the strength improvement is substantially
saturated, since the etching scattering deteriorates the accuracy
of the thickness of the beam 4, and thus a limit of the etching
amount is less than 2 .mu.m. Namely, the most suitable amount of
isotropic etching is 0.5-2.0 .mu.m.
Further, according to the measurement of the isotropic etching
amount in FIG. 10, as shown in FIG. 12 the silicon tip 2 of the
single crystal is prepared in a state such that a portion thereof
is masked and the etching is performed under the same conditions as
when manufacturing a sensor, and the step difference .delta. is
measured by a step difference meter to thereby obtain the etching
amount. Further, a state of a broken position in the silicon tip of
the single crystal where this isotropic etching is not performed is
shown in FIG. 13 and a state of a broken position in the silicon
tip of the single crystal where this isotropic etching is performed
is shown in FIG. 14. In the silicon tip where the isotropic etching
is not performed, as shown in FIG. 13, the beam 4 is broken from
the base, and in the silicon tip where the isotropic etching is
performed, as shown in FIG. 14, the beam 4 is broken away from the
base; namely, it was found that the break position is different.
FIG. 15 shows the etching amount and a ratio of two kind of broken
states (break at base, break away from base). As seen from FIG. 15,
when an isotropic etching more than 1 .mu.m is performed, the beam
4 is broken away from the base. Further, FIGS. 13 and 14 show
sketches of the broken parts.
Therefore, according to the present embodiment, to form a portion
of the silicon tip 2 of the single crystal as a thin beam 4 where
the piezo resistance layers 6-9 are formed, after the anisotropic
etching is performed, a 0.5 .mu.m-2.0 .mu.m isotropic etching of
the silicon tip is performed and as a result, the edge part of the
base in the beam 4 is rounded and the stress concentration thereof
is alleviated, and thus it is possible to produce an acceleration
sensor of a semiconductor with a beam 4 of the silicon tip 2 of the
single crystal having a high strength and a high sensitivity.
Namely, when the silicon tip of the single crystal is etched with
the anisotropic etching liquid alone, the base of the beam is very
easily broken. Especially, a sensor for detecting a very small
acceleration of less than 1 G, is very weak, and therefore, the
yield is low because the beam is thinned to about 20 .mu.m thick.
When the isotropic etching as in the present invention is
performed, it is possible to improve the strength of the beam.
As set forth above, according to the present invention, a
remarkable effect of producing an acceleration sensor of a
semiconductor having a semiconductor tip with a high strength and a
high sensitivity is obtained.
* * * * *